Using GUIDs in Primary Keys / Clusted Indexes

Question

I\'m fairly well versed in SQL server performace but I constanly have to argue down the idea that GUIDs should be used as the default type for Clusterd Primary Keys.

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Answer 1

If you are doing any kind of volume, GUIDs are extremely bad as a PK bad unless you use sequential GUIDs, for the exact reasons you describe. Page fragmentation is severe:

                 Average                    Average
                 Fragmentation  Fragment    Fragment   Page     Average 
Type             in Percent     Count       Size       Count    Space Used

id               4.35           7           16.43      115      99.89
newidguid        98.77          162         1          162      70.90 
newsequentualid  4.35           7           16.43      115      99.89

And as this comparison between GUIDs and integers shows:

Test1 caused a tremendous amount of page splits, and had a scan density around 12% when I ran a DBCC SHOWCONTIG after the inserts had completed. The Test2 table had a scan density around 98%

If your volume is very low, however, it just doesn't matter that much.

If you do really need a globally unique ID but have high volume (and can't use sequential IDs), just put the GUIDs in an indexed column.